The basal metabolic rate is the quantity of heat liberated per unit time by a living organism under prescribed resting conditions. Essentially all of the energy generated by the chemical reactions of the life process within the organism appears as heat energy, because little or no external work is done and having not eaten recently, the amount of energy being stored is zero or negligible. In a resting, fasting state, the basal metabolic activity can be measured as the rate of heat transfer from the body to the environment.
The BMR is usually determined under conditions of as complete mental and physical rest as possible, in a room with comfortable temperature, about 12 to 14 hours after the last meal. Determinations of BMR are useful, for example, in determining the caloric intake required by a given individual to maintain a steady weight. Different persons of the same weight and height can gain or lose weight on the same caloric intake, depending on their metabolic rates and physical activity.